American Indian (AI) populations experience disproportionately high rates of obesity and related chronic diseases (CD). Intervention trials aimed at modifying the food environment in AI communities have shown positive results, such as increasing the purchase and consumption of healthy foods. The current challenge is to facilitate the development of evidence-based policies in order to improve the sustainability of environmental interventions. The primary goal of the proposed research is to improve the food and physical activity (PA) environments in AI communities at multiple levels (worksites, schools, food stores) by working in partnership with tribal leaders to enact policies and programs that aim to reduce CDs. We will achieve this goal by developing, implementing, and evaluating a randomized controlled community-based trial which is supported or implemented by tribal leaders through policy. The study will take place in six AI communities located in Wisconsin and New Mexico. This project builds directly on our previous successful work in five AI communities where we conducted a multilevel obesity prevention trial (OPREVENT) targeting food stores, worksites and schools. This proposed trial (OPREVENT2) complements and expands upon the previous one, by fostering the development of relevant policies among tribal leaders which is key for sustainability. The specific aims of OPREVENT2 are: 1) To conduct formative research to describe tribal policy development and enactment in participating AI communities, in order to support tribal health policy makers to identify effective policies to sustain obesity and chronic disease prevention/reduction programs, by building capacity and collaborative partnerships; 2) To develop a community-based CD prevention program for AI communities informed by our earlier trials, formative research from Aim 1 and best-practices from the scientific literature; an 3) To evaluate the impact of the CD related policies and programs on adiposity, psychosocial factors and obesity risk behaviors, including dietary quality (e.g., fruit and vegetable servings), nutrient intake, and PA in a community- randomized controlled trial. The proposed study directly addresses NIH PAR-11-346 priorities by developing and testing an innovative multilevel obesity prevention strategy in AI communities that combines supportive policy with a culturally appropriate multilevel intervention. The proposed study is also locally generated and, therefore, more likely to be self-sustaining. We expect to impact the following areas of interest to the PAR: engagement of tribal leaders in developing and implementing CD policy, development of culturally appropriate community-based CD prevention programs, and increased availability of healthy foods, improvements in diet and PA, and reductions in adiposity.